If you mean for the achievement, I believe it's right after you finish the Brain Tumbler Experiment level (which requires you to also complete Sasha's and Milla's levels). I don't think you can get it after completing the Lake Oblongata level.

It's after Dogan's brain is removed. Head to the TV room and talk to all the kids. As you leave, Vernon will start to tell a scary story to impress the girls. Just instead of leaving, stay and listen to the story. :)